What happens to the grid when solar power goes dark during the eclipse?

The last solar eclipse that cast a shadow across the United States didn’t spark much interest from the nation’s electric utilities. That was back in 1991, before the sun became a significant contributor to the power grid.

But when the sky darkens during Monday’s solar eclipse, operators of the nation’s electric distribution systems will be watching closely for an expected fall-off in power production from solar panels. The blip will last about two hours.

A total eclipse will be experienced only in a 70-mile-wide ribbon stretching from Oregon to South Carolina. Still, the sky will darken significantly in a broad band north and south of the eclipse path, blocking much of the sun’s energy from reaching the earth. In the Philadelphia area, about 75 percent of the sunlight will be obscured.

PJM Interconnection, the regional power-grid operator, will be keeping a close watch on the eclipse’s impact on its 13-state territory.

The North American Electric Reliability Corp., the regulatory agency that ensures the security of the nation’s wholesale power supply, has been planning for this event for more than a year. So has PJM Interconnection Inc., the Valley Forge agency that manages the power grid in 13 states and the District of Columbia.

“We do not expect a major impact, but we’re really curious to see what does occur,” said Ken Seiler, PJM’s executive director of system operations. PJM is eager to see how the actual event measures up to its forecasts, he noted.

Across the country, solar generators now account for more than 42,000 megawatts of power, up from 5 MW in 2000. Solar power still accounts for just a small percentage of total electric generation, but Monday’s eclipse will serve as a reminder of the need for a diversity of energy sources, to fill in when intermittent power producers take a holiday.

Unlike a storm or an unplanned power-plant outage, the arrival time of an eclipse is well-known — one occurs when the moon is perfectly aligned to block out the sun — and the effect can be reasonably forecast, depending on cloud conditions. So grid operators are confident they can manage the window when the sun will go into hiding and then reemerge. But they hope to glean other insights during the episode.

Monday’s event will be a dress rehearsal for April 8, 2024, when another eclipse will occur in a northeasterly band stretching from Texas to Maine, directly affecting PJM’s territory, including northwestern Pennsylvania.

“We would anticipate to have a lot more solar generation at that time, so our hope is to use this as a pilot for the 2024 event, which is only seven years off,” said Seiler.

Monday’s eclipse will block nearly all the sun in the Carolinas and put Duke Energy’s power system to the test. Duke operates more than 30 solar farms in North Carolina.

Officials at Atlantic City Electric, which serves about 550,000 customers in the southern end of New Jersey and will experience a greater loss of sunlight than points north, are shrugging off the eclipse as inconsequential.

“We don’t have any operational concerns,” said Frank Tedesco, an Atlantic City Electric spokesman. “Our people equate it to a very cloudy day — the eclipse will have about the same impact as a large storm moving through.”

If Monday is a sunny day, the regional grid will experience a decline of as much as 2,500 MW of power generation at the eclipse’s midafternoon peak, PJM’s planners say. At 3 p.m. on a typical August afternoon, PJM’s load is about 130,000 MW, so the loss of 2,500 MW is less than 2 percent.

Because PJM has more generation capacity available than it needs on a peak day, even if Aug. 21 is sweltering and air-conditioning systems are straining under the load, the system can manage the loss of the solar power, Seiler said.

“We’re capacity-rich in the amount of generation we have available to us, so we have plenty of reserves,” he said. “If we ran into a situation, we could always lean on our neighbors and secure some help from them.”

Electricity demand for lighting increased during that eclipse — not surprising, since there was less sunlight. But the European Network of Transmission System Operators for Electricity also measured some suppression of demand because so many people went outdoors to watch the eclipse. The European grid operator said wind generation, too, dropped during the eclipse, by about 10 percent in Britain, demonstrating the sun’s effect on winds.

In advance of the 2015 eclipse, European grid operators disconnected some large solar farms to reduce the need to match up other power sources during the rapid fall-off in solar output. During this eclipse, however, not all U.S. grid solar operators plan to disconnect: Public Service Electric & Gas Co., which operates nine solar farms in New Jersey, says its facilities will chug away as if it’s a cloudy day.

“The August 21st eclipse could result in lower PSE&G solar generation that is similar to what we would experience during overcast weather conditions,” said Todd Hranicka, the company’s director of solar energy. “There will be no impact on grid reliability since any reduction in solar generation will be easily offset with traditional base generation.”

A big wild card in the projections is what happens to customers who have individual rooftop solar installations that are “behind the meter,” which means the utility can’t turn them on and off. But in PJM’s territory, about 2,000 MW of the expected 2,500-MW impact will come from individual customers who are expected to demand more power from the grid when their rooftop systems take a nap.

The eclipse’s impact will be measured in the fairly rapid increase in demand for power from solar customers, followed by a comparable increase in power production as customers’ solar panels reawaken.

If Monday is overcast, the impact on the grid will be less dramatic because solar generation’s contribution will already be substantially reduced.

“Our hope frankly is that we have a sunny day and we can really see the obscurity level and how that correlates to what our studies have been, just to kind of marry that up,” said Seiler.

On any given day, PJM copes with the normal fluctuations in demand by dispatching power plants to ramp output up or down. It typically keeps as much reserve power ready to match the output of its largest power plant, should it shut down in an emergency.

On Monday, PJM will keep a little more power on standby — it’s called “spinning reserve,” which are power plants that are operating at less than maximum capacity. The reserve units include natural gas plants and run-of-river hydroelectric plants or pumped-storage hydroelectric facilities that can be ramped up or down in seconds.

What happens to the grid when solar power goes dark during the eclipse?